The problem of accurately drilling holes in large workpieces such as aircraft wing and fuselage panels and other types of structures has been an ongoing challenge in the aircraft industry as well as other industries, and one for which a completely satisfactory solution applicable to a wide range of workpiece configurations has not heretofore been provided. Large fixed-monument machines such as five-axis drilling machines can be used for some types of workpieces, but these machines are quite expensive to procure and operate. In contrast, a relatively low-cost solution to the above-noted problem that has been developed by the assignee of the present application and others is to mount an automated drill or other working module on a track that is mounted to the workpiece. The drill or module is supported on a carriage that travels along the track, which is formed by a pair of parallel rails mounted on the workpiece. For examples of such devices, see U.S. Pat. No. 4,850,763, assigned to the assignee of the present application and incorporated herein by reference, and U.S. Pat. No. 3,575,364.
In the above-noted patents, however, the embodiments illustrated and described were applied to workpieces that did not have compound-contoured surfaces. As used herein, the term “compound-contoured” is used to denote a surface having curvature in more than one direction. On such a compound-contoured surface, it is not possible in general to lay a pair of straight, flexible rails such that the rails conform to the surface contour and are the same distance apart at all points along the rails. Thus, the surface of a sphere is an example of a compound-contoured surface, because in the general case the spacing between a pair of flexible rails laid on the surface will vary. In contrast, a circular cylinder does not have a compound-contoured surface, because the rails can be laid in either circumferential, axial, or helical directions and the spacing between them can be constant. In U.S. Pat. No. 3,575,364 noted above, a pair of flexible rails are mounted in the circumferential direction around a circular cylindrical workpiece. It will be appreciated that the rails can be perfectly parallel in such an arrangement, because the cylindrical surface is a simple-contoured surface. The rails in the '364 patent are made flexible so that they can be conformed to a variety of surfaces, but even such flexible rails cannot be made exactly the same distance apart at all points along the rails when they are mounted on a compound-contoured surface. Furthermore, rails mounted along two different paths on a compound-contoured surface will twist differently from each other because of the different directions of the surface normals along the two paths. This can make it difficult to traverse a carriage along the rails and maintain good accuracy of carriage positioning.
It is possible to mount a pair of spaced rails on a compound-contoured surface such that the rails are the same distance apart at all points along the rails, but only by custom-designing the rails for the particular workpiece surface. If such custom-designed rails were used on a differently contoured surface, they would not be the same distance apart at all points. While it is highly desirable to be able to traverse a drill or other machine component on a pair of rails mounted on a compound-contoured surface, it is also desirable to be able to use the same apparatus on a wide variety of surface contours, including simple- and compound-contoured surfaces.